This disclosure relates, in various embodiments, to electrophotographic imaging members. The imaging members described herein can be used as photosensitive members, photoreceptors or photoconductors useful in electrophotographic systems, including printers, copiers, other reproductive devices, and digital apparatuses. More particularly, the imaging members of this disclosure do not require an anti-curl back coating to maintain flatness, etc., and comprise at least a flexible substrate and a layer comprising a charge transport material having certain characteristics. The disclosure also relates to methods of imaging utilizing such imaging members.
Electrophotographic imaging members, such as photoreceptors or photoconductors, typically include a photoconductive layer formed on an electrically conductive substrate or formed on layers between the substrate and photoconductive layer. The photoconductive layer is an insulator in the dark, so that during machine imaging processes, electric charges are retained on its surface. Upon exposure to light, the charge is dissipated, and an image can be formed thereon, developed using a developer material, transferred to a copy substrate, and fused thereto to form a copy or print. Electrophotographic imaging members are typically in either a flexible belt configuration or rigid drum form. Flexible imaging member belts may either be seamed or seamless belts. However, for reasons of simplicity, the disclosure hereinafter will focus only on electrophotographic imaging members in a flexible belt configuration.
For typical negatively-charged flexible electrophotographic imaging member belts, the outermost exposed photoconductive layer is a charge transport layer. Therefore, under normal machine service conditions, the charge transport layer is repeatedly subjected to various machine subsystems mechanical interactions and constantly exposed to corona effluents (emitted from a charging device) and other volatile organic compound (VOC) species/contaminants. Mechanical interactions against imaging member cause the charge transport layer to develop wear, abrasion, and scratch. Wear reduces the charge transport layer thickness, effectively changing the charging field strength. Scratches manifest themselves as printout defects. Exposure to corona effluents and chemical contaminants gives rise to charge transport layer material degradation and lateral charge migration (LCM) problems. Charge transport layer material degradation facilitates the premature onset of layer cracking and LCM. All of these physical and mechanical failures impact copy image quality and cut short the intended functional life of an electrophotographic imaging member belt, requiring frequent and costly belt replacement.
In a service environment, a flexible imaging member belt, mounted on a belt supporting module, is exposed to repetitive electrophotographic image cycling which subjects the outer-most charge transport layer to mechanical fatigue as the imaging member belt bends and flexes over the belt drive roller and all other belt module support rollers, as well as sliding bend contact above each backer bar's curving surface. This repetitive imaging member belt cycling leads to a gradual deterioration in the physical and mechanical integrity of the exposed outer charge transport layer leading to premature onset of fatigue charge transport layer cracking. The cracks developed in the charge transport layer as a result of dynamic belt fatiguing manifest themselves as copy printout defects which adversely affect image quality. In essence, the appearance of charge transport cracking cuts short the imaging member belt's intended functional life.
Many advanced imaging systems are based on the use of a flexible imaging member belt mounted over and around a belt support module design employing small diameter belt rollers. Although small diameter for belt module support rollers are used to provide easy paper self-stripping, the benefit of easy paper copy stripping negated by the large charge transport layer bending strain induced during dynamic fatigue belt flexing/bending motions over the small belt module support roller during image member belt machine functioning. The large bending strain induced by each small belt support module roller aggravates the mechanical problems that lead to early onset of charge transport layer cracking. Moreover, charge transport layer cracking frequently occurs at those belt segments parked over the support rollers during prolonged machine idling or overnight/weekend shut off periods as a result of exposure to residual corona effluents and airborne chemical contaminants. The early onset of charge transport layer cracking is a serious issue that impacts copy printout quality.
For typical negatively-charged imaging member belts, such as flexible photoreceptor belt designs, there are multiple layers comprised of a flexible supporting substrate, a conductive ground plane, a charge blocking layer, an optional adhesive layer, a charge generating layer, and an outermost exposed charge transport layer. The charge transport layer is usually the last layer to be coated and is applied by solution coating followed by drying at elevated temperatures, then cooling to ambient room temperature. When a production web stock of several thousand feet of coated multilayered photoreceptor material is obtained after finishing the charge transport layer coating and drying/cooling process, upward curling of the multilayered photoreceptor is observed.
This upward curling has been determined to be the consequence of thermal contraction mismatch between the applied charge transport layer and the substrate support. Because the charge transport layer in a typical conventional photoreceptor device, using polycarbonate as binder, has a coefficient of thermal contraction approximately 3.7 times greater than that of the flexible substrate support (usually a polyethylene naphthalate or a polyethylene terephthalate), the charge transport layer has a greater dimensional contraction than that of the substrate support as it cools down to ambient room temperature. The resulting internal tension strain in the charge transport layer causes the photoreceptor to exhibit upward curling. If unrestrained, the photoreceptor would spontaneously curl upwardly into a tube. To offset this curl and keep the photoreceptor web stock flat, an anti-curl back coating (ACBC) is applied to the backside of the flexible substrate support, opposite to the side having the charge transport layer.
Although an ACBC is required to keep the photoreceptor flat, its application represents more than just an additional coating step. It increases the labor and material cost and also decreases daily photoreceptor production through-put by about 25%. Moreover, the ACBC coating application frequently results in photoreceptor production yield lost due to web stock scratching damage caused by handling. In addition, the use of an ACBC has also been determined to cause an internal built-in strain of about 0.28% in the charge transport layer. This internal strain is cumulatively added onto each photoreceptor bending induced strain as the photoreceptor belt flexes over a variety of belt module support rollers during dynamic belt cycling function within a machine. Consequently, this internal built-in strain compounds and exacerbates the fatigue bending strain in the charge transport layer, causing early onset of charge transport layer cracking.
Seamed flexible photoreceptor belts are fabricated from sheets cut from an electrophotographic imaging member web stock having anti-curl back coating. The cut sheets are generally rectangular in shape. All edges may be of the same length or one pair of parallel edges may be longer than the other pair of parallel edges. The sheet is formed into a belt by joining the overlapping opposite marginal end regions of the sheet. A seam is typically produced in the overlapping opposite marginal end regions at the point of joining. Joining may be effected by means such as welding (including ultrasonic processes), gluing, taping, or pressure/heat fusing. However, ultrasonic seam welding is generally the preferred method of joining because it is rapid, clean (no application of solvents) and produces a thin and narrow seam. The ultrasonic seam welding process involves a mechanical pounding action of a welding horn which generates a sufficient amount of heat energy at the contiguous overlapping marginal end regions of the imaging member sheet to maximize melting of one or more layers therein. A typical ultrasonic welding process is carried out by pressing down the overlapping ends of the flexible imaging member sheet onto a flat anvil and guiding the flat end of the ultrasonic vibrating horn transversely across the width of the sheet and directly over the overlapped junction to form a welded seam having two adjacent seam splashings consisting of the molten mass of the imaging member layers ejected to either side of the welded overlapped seam.
These seam splashings of the ejected molten mass comprise about 40% by weight of the anti-curl back coating material. Seam splashings are undesirable projection features because they interfere with cleaning blade action, causing blade damage and wear which leads to premature loss of cleaning efficiency. The seam splashing present at the back side of the photoreceptor belt has also been found to physically interact with the belt support module roller, affecting the photoreceptor belt's delicate motion/cycling speed during an imaging process and impacting toner image formation as reflected in the copy printout quality.
Another disadvantage of an ACBC is that the ACBC is in constant mechanical interaction with the machine belt support rollers and backer bars; this causes substantial wear of the ACBC. The ACBC may also be susceptible to degradation by ozone attack, which also accelerates wear. ACBC wear generates dust inside the machine cavity and reduces the thickness of the anti-curl layer, diminishing its ability to keep the photoreceptor belt flat. This upward belt curling, caused by loss of ACBC thickness, produces significant surface distance variation between the photoreceptor belt surface and the machine charging device; this variation causes non-uniform charging density over the photoreceptor belt surface, degrading copy printout quality.
In addition, photoreceptor belt upward curling under dynamic belt functioning conditions causes the belt to physically interact/interfere with the xerographic subsystems, particularly in those machines employing a hybrid scavengeless development (HSD) or hybrid jumping development (HJD) subsystem. This interaction leads causes undesirable artifacts which manifest themselves as printout defects.
With the noted undesirable traits described above, it is clear that flexible seamed photoreceptor belts which do not require an ACBC can reduce the belt unit manufacturing cost, increase belt yield and daily production throughput, provide extended service life, and suppress of early onset of charge transport layer cracking by eliminating internal strain.
In U.S. Pat. No. 5,089,369 to R. Yu, issued on Feb. 18, 1992, the disclosure of which is fully incorporated herein by reference, an electrophotographic imaging member having a supporting substrate and a charge generating layer, the supporting substrate material having a thermal contraction coefficient which is about the same as that of the charge generating layer, is disclosed. Substrate materials that have a thermal contraction coefficient value from about 5.0×10−5/° C. to about 9.0×10−5/° C. are used in combination with a benzimidazole perylene charge generating layer.
U.S. Pat. No. 5,167,987 to R. Yu, issued on Dec. 1, 1992, discloses a process for fabricating an electrostatographic imaging member including providing a flexible substrate comprising a solid thermoplastic polymer, forming an imaging layer coating including a film forming polymer on the substrate, heating the coating and substrate, cooling the coating and substrate, and applying sufficient predetermined biaxial tensions to the substrate while the imaging layer coating and substrate are at a temperature greater than the Glass Transition Temperature (Tg) of the imaging layer coating to substantially compensate for all dimensional thermal contraction mismatches between the substrate and the imaging layer coating during cooling of the imaging layer coating and the substrate, removing application of the biaxial tensions to the substrate, and cooling the substrate whereby the final hardened and cooled imaging layer coating and substrate are free of internal stress and strain. The disclosure of the '987 patent is also fully incorporated herein by reference.
U.S. Pat. No. 4,983,481 to R. Yu, issued on Jan. 8, 1991, discloses an imaging member without an anti-curl backing layer having improved resistance to curling. The imaging member comprises a flexible supporting substrate layer, an electrically conductive layer, an optional adhesive layer, a charge generating layer, and a charge transport layer, the supporting substrate layer having a thermal contraction coefficient substantially identical to the thermal contraction coefficient of the charge transport layer. The supporting substrate may be a flexible biaxially oriented layer. The disclosure of this patent is further fully incorporated herein by reference.
While the above mentioned curl-free flexible imaging members having no ACBC may be useful for their intended purpose of resolving specific problems, resolution of one problem has often been found to create new ones. For example, the selection of a supporting substrate having thermal contraction matching that of the charge transport layer has been observed to be susceptible to attack and damage by solvents used in the charge transport layer coating solution, rendering the imaging member useless. Other substrate supports have good thermal contraction matching properties but also have inherently low glass transition temperatures (Tg) which are not suitable for imaging member fabrication. Applying biaxial tensioning stress onto imaging members maintained at a temperature slightly above the glass transition temperature (Tg) of the charge transport layer is a costly and cumbersome batch process.
There continues to be a need for improved imaging members, especially flexible imaging member belts, which do not have an ACBC, wherein the layer comprising the charge transport material has little or no internal built-in strain, is less susceptible to cracking induced by fatigue bending, and is less susceptible to material failure from exposure to corona effluents and airborne chemical contaminants.